Caltech Question of the Week: Why Does There Need To Be Water On a Planet or Moon To Have Life?

Water is a tremendously important ingredient in that it's a very good solvent and a very good medium for chemical reactions. It's also very common.

Water is nearly everywhere in the solar system, it's easy to make from two ingredients (hydrogen and oxygen) that are both very common throughout the solar system and the universe, and it can exist in some truly harsh environments.

In fact, the harsh Earth environments in which liquid water is found give us good reason to think that water could be associated with extraterrestrial life. Anywhere you look on Earth, no matter how inhospitable the environment seems, liquid water apparently always harbors life of some sort. This is true for subterranean rocks as well as superheated ocean vents. So since you can find life in such Earth environments, it makes sense that life could also exist in harsh environments elsewhere.

Thus, water is a great substance for conducting chemistry. And life is very sophisticated chemical activity.